1) Field of the Invention
The present invention relates to non-destructive inspection and, more particularly, to non-destructive inspection of a structure for defects using an inspection system in conjunction with a data acquisition system.
2) Description of Related Art
It is frequently desirable to inspect structures to identify defects or flaws, such as cracks, discontinuities, voids, or porosity, which could adversely affect the performance of the structure. Non-destructive inspection (“NDI”) of structures is typically utilized to thoroughly examine a structure without harming the structure or requiring significant disassembly. Non-destructive inspection is typically preferred to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure. NDI is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, NDI is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to, or defects (flaws) in, the structure. Inspection may be performed during manufacturing or after the completed structure has been put into service, including field testing, to validate the integrity and fitness of the structure.
Various types of sensors may be used to perform NDI. One or more sensors may move over the portion of the structure to be examined, and receive data regarding the structure. For example, a pulse-echo (PE), through-transmission (TT), or shear-wave sensor may be used to obtain ultrasonic data, such as for thickness gauging, detection of laminar defects and porosity, and/or crack detection in the structure. Resonance, pulse-echo, or mechanical impedance sensors are typically used to provide indications of voids or porosity, such as in adhesive bondlines of the structure. High resolution inspection of aircraft structure is commonly performed using ultrasonic testing (UT) to provide a plan view image of the part or structure under inspection. Data acquired by sensors is typically processed and then presented to a user via a display as a graph of amplitude of the received signal. To increase the rate at which the inspection of a structure is conducted, a scanning system may include arrays of inspection sensors, i.e., arrays of transmitters and/or detectors.
Various coatings, such as paint, primer, adhesive layers, or corrosion inhibiting compounds, may be applied to metallic structures, complicating NDI of the structures. Generally, NDI techniques used on thicker structures are incompatible and unreliable for inspecting thinner structures having coatings. Thin materials may have relatively small flaws that would be acceptable in a thicker structure but that have a disproportionately large affect on the quality of a thin structure and, therefore, are desirably detected. With the detection of such smaller flaws, more noise and other spurious signals are also detected and analyzed which can lead to false results and render the NDI technique unreliable. Moreover, the complications upon NDI posed by a coating are also generally more pronounced with respect to thinner structures.
Generally, a robust NDI technique has an acceptable signal-to-noise ratio (i.e., 3-to-1) and is capable of identifying flaws, at least larger flaws, rather than spurious signals. However, a robust NDI technique may miss some smaller flaws since the signals generated by the smaller flaws may be considered noise. A sensitive NDI technique has a lower signal-to-noise ratio so as to identify smaller flaws, but is more prone to identify spurious signals as being indicative of a potential flaw since the spurious signals will sometimes exceed the signal threshold that has been established to define a flaw. Adding gain to a sensitive NDI technique in an attempt to detect smaller flaws amplifies spurious signals, as well as flaws, which leads to false rejections of structures. Conversely, reducing gain to a sensitive NDI technique provides a more robust inspection because less false signals will be identified to be indicative of flaws, but the inspection is less sensitive and not as many flaws, especially smaller flaws, will be identified.
Previous NDI techniques determined the attenuation of the worst case coating condition, generally the thickest possible coating of the most attenuative material, and compensated for the attenuation by adding gain. However, in practice this often leads to excessive gain settings on a structure that does not have the worst case coating condition. Excessive gain settings to offset attenuation created by a coating magnify the noise level, causing irrelevant signals to be mistaken for defects, particularly for thinner structures. As such, unwarranted repairs and a loss of confidence in the inspection technique may result. Moreover, paint stripping of the structure prior to an inspection due to the difficulty in inspecting thinly coated metallic structures has been an expensive alternative. However, without a reliable inspection system, this may be one of the few alternatives for inspecting the coated structure.
One type of NDI uses ultrasonic shear wave techniques. However, NDI using ultrasonic shear wave techniques causes several problems when inspecting thin structures having a coating applied thereon. The coating dampens the ultrasonic beam entering the structure as the ultrasonic beam bounces between opposing surfaces of the structure, which attenuates the ultrasonic energy until there is insufficient ultrasonic energy to detect flaws. In addition, ultrasonic shear waves entering the structure can convert to less well-behaved modes resulting in unpredictability in locating flaws within the structure. Moreover, modifying the coating thickness generates nonlinear attenuation effects (i.e., no change, small change, or large change in attenuation) such that modeling these effects is difficult. Without an accurate model, it is more difficult to use a gain compensation formula as would otherwise be employed by ultrasonic inspection techniques to account for coating thickness variations. Such a formula would also require a priori knowledge of the coating thickness on the structure to be tested, which is generally not known.
It would therefore be advantageous to provide an inspection system that is more reliable for inspecting thin structures having a coating. It would also be advantageous to provide an inspection system that is capable of compensating for attenuation and adjusting gain to compensate for a coating present on the structure. It would be further advantageous to provide an inspection system that is practical and economical.